1. Field of the Invention
The present invention relates to a transport apparatus for transporting an object to be transported such as a semiconductor wafer. More particularly, the present invention relates to a transport apparatus suitable for a semiconductor manufacturing system or the like that includes one or more processing chambers.
2. Description of the Related Art
A transport apparatus has been conventionally proposed, which brings a substrate into a processing chamber and carries it out from the processing chamber in a semiconductor manufacturing system (see the publication of Japanese Patent No. 3204115, for example). In the processing chamber, various processes can be performed.
The conventional transport apparatus has first, second, and third arms. Those arms respectively have three shafts that are coaxially arranged, and can be rotated independently of each other.
Each of the rotary shafts which drive each arms is linked to a reduction gear provided in an arm-rotating motor by means of a belt.
A base end of a first driven arm is rotatably linked to a top end of the first arm. A base end of a second driven arm is rotatably linked to a top end of the second arm. A first substrate holder is attached to top ends of the first and second driven arms.
Moreover, a base end of a third driven arm is rotatably linked to a top end of the third arm. A base end of a fourth driven arm is rotatably linked to the top end of the second arm in such a manner that the fourth driven arm can be coaxially rotated with respect to the second driven arm. A second substrate holder is attached to the top ends of the second and fourth driven arms.
However, because the rotary driving shafts are linked to the reduction gear provided in the arm-rotating motor by means of belts in the conventional transport apparatus, a rotary force of the reduction gear is not properly transmitted to the respective rotary shafts in the case where tensions of the belts are changed by change in the surrounding temperature, change of the material with time, or the like. In some cases, hysteresis occurs in rotation of each rotary shaft due to normal rotation and reverse rotation of the arm-rotating motor. In those cases, the substrate placed on the substrate holder is not transported to a correct position.
In addition, it is necessary to use a motor that can generate a large torque as the arm-rotating motor because a transmitting force is consumed in the reduction gear. This makes the size of the transport apparatus larger and increases the fabrication cost of the transport apparatus.
Moreover, in case of using the conventional transport apparatus that is attached to a vacuum chamber such as a semiconductor manufacturing system, the transport apparatus has to have an airtight structure. Thus, it is necessary to provide shaft seal mechanisms between adjacent rotary driving shafts and between an outermost rotary driving shaft and a casing of a rotary shaft unit in order to flexibly link them in an airtight manner. The shaft seal mechanism is formed by magnetic fluid, an O-ring, or Wilson seal, for example. However, the shaft seal mechanism serves as a resisting force to the rotation of each rotary driving shaft. Thus, in case of high-speed rotation, it is necessary to use a motor that can generate a large torque as the arm-rotating motor. This increases the size and the fabrication cost of the arm-rotating motor and also makes the transport apparatus larger.
Furthermore, the conventional transport apparatus has many components such as the rotary driving shafts, arm-rotating motor, and reduction gear. Thus, the fabrication cost of the conventional transport apparatus is high. The maintenance cost is also high because the components of the transport apparatus include many slidable parts.
Furthermore, the conventional transport apparatus detects an angle of rotation of a motor shaft of the arm-rotating motor by means of an angle detector and controls the angle of rotation. That is, an angle of rotation of each rotary shaft is not directly detected. Thus, it is not possible to detect whether or not each rotary shaft is rotated in an appropriate manner in accordance with a rotation instruction supplied to the arm-rotating motor. Therefore, the substrate placed on the substrate holder may not be transported to a correct position.
In the above conventional transport apparatus, when the first and second substrate holders are located at the closest positions to the coaxial rotary shaft and face toward each other in a vertical direction (i.e., they are in a rotatable state), they are located on the same side of a straight line passing through the coaxial rotary shaft.
Thus, in the case where the transport apparatus performs a rotation while a large substrate is placed on the substrate holder, the distance from an end of the substrate to the center of rotation becomes larger and the rotation radius of the transport apparatus also becomes larger.
When the transport apparatus having such a large rotation radius is incorporated into a semiconductor manufacturing system, a central chamber for accommodating the transport apparatus therein becomes larger. Thus, an area required for installing the entire semiconductor manufacturing system becomes larger.
In addition, in the conventional transport apparatus having such a large rotation radius, a large centrifugal force is applied to the substrate on the substrate holder during the rotation. That centrifugal force may change the position of the substrate on the substrate holder when the rotation speed of the conventional transport apparatus increases. This may prevent the transport of the substrate.